This invention relates to the reduction in content of one or more metallic impurities from molten metals and more particularly to the reduction in content of copper from ferrous metal scrap.
For the production of steel in the United State, the increased use of ferrous metal scrap for steel would have a number of advantages. Usually, scrap is a less expensive source of ferrous metal compared to many ores. Also, scrap is often readily available and does not require extensive reduction processes before being added to the melt. In addition, the increased use of scrap would reduce the problem of storing or disposal of excess scrap.
One of the limitations in the use of scrap for steel production involves the content of impurities in the scrap. Copper is one impurity capable of causing particular problems in the production and use of stel products. In the production of steel, a copper content in the order of 0.5 wt.% has been reported to cause surface cracking during reheating and rolling. In some commercial uses such as tire cord, copper content is limited to a maximum of 0.05 wt.% with lower values being desirable. A common scrap melt may contain a copper level between 0.2 and 0.6 wt.% which would necessitate a significant use of high-grade and more expensive scrap with low copper in the charge.
U.S. Pat. No. 2,512,578, U.S. Pat. No. 3,441,404, U.S. Pat. Nos. 1,425,701 and 1,377,217 provide some background details on the problem of copper in ferrous metals and methods of removal. In U.S. Pat. No. 2,512,578, (which involves both the removal of copper and the added sulfur), sulfur is added in small amounts to the molten ferrous metal to convert the copper to copper sulfide with the resulting sulfur content in the metal scrap being limited to a low value. In a subsequent stage, a metal sulfide slag is contacted with the molten scrap to extract copper sulfide. Sodium sulfide, potassium sulfide and aluminum sulfide are disclosed as slag components.
In U.S. Pat. No. 3,441,404, sulfur and a halide-containing compound are added to the melt to remove a copper-sulfur compound during distillation of the halide. In U.S. Pat. No. 1,425,701, metal sulfides such as iron sulfide are disclosed for converting the copper to copper sulfide. In U.S. Pat. No. 1,377,217, a sulfidizing material is added to scrap iron in a furnace to form copper sulfide which is removed by processing in a leaching tank.
As disclosed above, the reduction in copper content involves both the conversion of copper to copper sulfide or other compounds and its removal by extraction, distillation or other techniques. Each step of the process has particular problems. In U.S. Pat. No. 2,512,578, the sulfur addition to molten scrap is limited to provide a low sulfur content in resulting ferrous metal. Extractions by a metal sulfide slag are limited by the difficulty of achieving a reasonable degree of extraction while avoiding significant dissolution of the sulfide slag in the molten metal.
Accordingly, one object of the invention is the increased use of metal scrap and particularly ferrous metal scrap in the industrial production of metal products. A second object of this invention is a process for reducing the content of impurities in molten metals or in solid metal scrap containing separate copper bodies (wires, connectors, bus bars, etc.) which cannot be economically separated otherwise. Another object of the invention is a process for reducing the copper content of ferrous metal scrap. An additional object of the invention is a process for the extraction of copper as a sulfide from ferrous metal scrap while limiting any significant dissolution or reaction of the slag in the molten or solid ferrous metal. A further object of the invention is a process for the reduction of the copper content of ferrous metal scrap together with a reduction in the process steps associated with copper sulfide formation and removal. These and other objects will become apparent from the following disclosure.